1. Technical Field
The present disclosure generally relates to a conductive member employed as a charging member or a transfer member provided near an image carrier, a process cartridge including the conductive member, and an image forming apparatus employing an electrophotographic method, such as a copier, a laser-beam printer, or a facsimile machine, and including the conductive member.
2. Related Art
A conductive member may be employed as a charging member to charge an image carrier (e.g., photoconductor) or a transfer member to transfer toner on an image carrier in an image forming apparatus employing an electrophotographic method, such as a copier, a laser-beam printer, or a facsimile machine.
The following is a description of a configuration and action of a process cartridge of an image forming apparatus (e.g., printer) employing a conductive member as a charging member.
FIG. 1 is a schematic view of an image forming apparatus employing an electrophotographic method. In FIG. 1, the image forming apparatus includes an image carrier 11 (i.e., electrostatic latent image carrier), a charging member 12 (i.e., charging roller) that implements charging provided near or in contact with the image carrier 11, and an exposure 13 by a laser light or a reflected light from a manuscript. Further, the image forming apparatus includes a toner carrier 14 (i.e., developing roller) that attaches a toner 15 to an electrostatic latent image on the image carrier 11 and a transfer member 16 (i.e., transfer roller) that transfers a toner image on the image carrier 11 to a recording medium 17. The image forming apparatus also includes a cleaning member 18 (i.e., blade) for cleaning the image carrier 11 after transfer, a developing device 20, and a cleaning device 21. The image forming apparatus also includes discarded toner 19 generated by removal of residual toner on the image carrier 11 with the cleaning member 18.
In FIG. 1, functional units are illustrated for description and other units are omitted for simplicity.
The image forming apparatus forms an image as follows.
1. The charging roller (i.e., charging member) charges a surface of the image carrier to a desired potential.
2. An exposure device irradiates an image light onto the image carrier to form an electrostatic latent image corresponding to a desired image on the image carrier.
3. The developing roller develops the electrostatic latent image with toner and forms a toner image (visible image) on the image carrier.
4. The transfer roller transfers the toner image from the image carrier to the recording medium.
5. The cleaning device cleans non-transferred residual toner on the image carrier.
6. The transfer roller conveys the recording medium with the transferred toner image to a fixing device not shown in FIG. 1. The fixing device fixes the toner image on the recording medium with heat and pressure.
A desired image is formed on the recording medium by repeating steps 1 to 6.
In the above description, a contact charging method in which the charging roller contacts the image carrier is described as a charging method employing the charging roller.
However, the contact charging method has the following problems.
1. Trace of the charging roller (which occurs as a material constituting the charging roller seeps out and adheres to a surface of a charging target body).
2. Charging sound (which occurs due to vibration of the charging roller contacting the charging target body when an alternating-current voltage is applied to the charging roller).
3. Decline in charging performance due to the toner on the image carrier adhering to the charging roller (which occurs, in particular, as the above-described seeping out is likely to cause such toner adherence).
4. Adherence of the material constituting the charging roller to the image carrier.
5. Permanent deformation of the charging roller caused when the image carrier stops for a long period.
In view of the above-described problems, for example, JP-H03-240076-A, JP-H04-358175-A, and JP-H05-107871-A proposes a proximity charging method in which the charging roller is provided near the image carrier. JP-2005-091818-A also describes a configuration in which space maintaining members are press fit into both ends of a resistance adjustment layer and simultaneous processing (removal processing) of the space maintaining members and the resistance adjustment layer to allow precise control of a space.
The required characteristics of the charging roller employed in the proximity charging method differs from the required characteristics of the above-described charging roller employed in the contact charging method. The charging roller generally used in the contact charging method has a configuration of an elastic body, such as vulcanized rubber, coating the circumference of a metal core. In the contact charging method, the charging roller needs to uniformly contact the image carrier to charge the image carrier uniformly.
If the charging roller formed of the above-described elastic body is used in the proximity charging method, the following problems occur.
1. It is necessary to bring the charging roller into proximity of the image carrier with a space between the charging roller and the image carrier being maintained by the space maintaining members, such as spacers, provided in non-image forming areas at both ends of the charging roller. However, when the charging roller formed of the elastic body is employed, deformation of the elastic body makes it difficult to maintain a uniform space. As a result, charging potential variation and unevenness of image due to the charging potential variation may occur.
2. The vulcanized rubber forming the elastic body is susceptible to wear and deformation after long-term use, which results in fluctuations of the space over time.
To overcome the above-described problems, it is conceivable to employ a thermoplastic resin, which is a non-elastic body, for the charging roller employed in the proximity charging method. Such a configuration allows the space between the charging roller and the image carrier to be maintained uniform.
A mechanism for charging the surface of an image carrier drum with the charging roller is known to be micro discharge between the charging roller and the image carrier drum according to Paschen's Law. For the image carrier drum to acquire the function to maintain a desired charging potential, the resistance value of the thermoplastic resin needs to be controlled within a semi-conductivity range of approximately 106 Ωcm to 109 Ωcm.
As a method of controlling the resistance value, the method of mixing and dispersing a conductive pigment such as carbon black within a thermoplastic resin matrix is typically employed. However, when the resistance adjustment layer employing the conductive pigment is set within the semi-conductivity range, the resistance value may greatly fluctuate and image defects, such as partial charge failure, may occur.
Another method to control the resistance value is employing an ion conductive material such as electrolytic salt. The ion conductive material disperses at a molecule level within a resin matrix. Compared to the case of dispersing the conductive pigment, the resistance value variation is small and decline in image quality due to the resistance value variation is overcome.
However, low molecular weight ion conductive material such as electrolytic salt has a tendency to bleed out on the surface of the resin matrix. When the ion conductive material bleeds out onto the surface of the charging roller, a fixation of the toner is generated and image defect occurs.
To overcome the bleeding out of the ion conductive material, employing a solid state polymer ion conductive material such as a polyether ester amide is conceivable. By employing the solid state polymer ion conductive material, the ion conductive material is dispersed and fixed within the resin matrix and bleeding out of the ion conductive material onto the surface is less likely. However, the addition of only the polymer ion conductive material makes the resistance value of the resistance adjustment layer high and controlling the resistance value to the semi-conductivity range is not possible. Therefore, a method of adding the electrolytic salt to the polymer ion conductive material is employed to impart sufficient conductivity. Specific examples of widely used electrolytic salts include perchlorates (e.g., sodium perchlorate, lithium perchlorate) and organic anion salts containing fluorine (e.g., lithium trifluoromethanesulfonate).
However, the polymer ion conductive material itself generally has a high water absorbing property. Thus, when the polymer ion conductive material is added to a material, the volume expansion rate (i.e., swelling properties) by water absorption of the material becomes high. Water is in the intervening air of conduction path. Accordingly, when the polymer ion conductive material is employed in the resistance adjustment layer of the charging roller in the proximity charging method, the change of the space between the charging roller and the image carrier due to the environment becomes large and charging properties decline leading to image defects.
More specifically, under high temperature and high humidity environments, the charging roller swells and the space between the charging roller and the image carrier declines. In extreme cases, the charging roller and the image carrier contacts each other. Using the charging roller under high temperature and high humidity environments for a long period results in the transfer and adherence of discharge products on the image carrier to the charging roller side, and conductivity at sections to which the discharge products adhere to decline. As a result, image defects occur. In addition, under low temperature and low humidity environments, the space becomes large and discharge from the charging roller to the image carrier becomes non-uniform leading to image defects.
It is possible to reduce swelling properties of the charging roller by increasing the mixing ratio of the thermoplastic resin with insulation properties within the resistance adjustment layer, or by adjusting the ratio of functional groups contributing to water absorbability of the polymer ion conductive material. The material of the charging roller obtains low water absorption properties reducing swelling. However, at the same time, resistance increase occurs and necessary conductivity for the charging roller cannot be obtained.
JP-2009-134050-A describes a technology to mix a fiber formed of a polymer having an aromatic skeleton in the molecule and having the same insulation properties as the thermoplastic resin that does not melt when forming the charging roller. With the technology, reduction of water absorbability is obtained without generating resistance increase as in the case of increasing the mixing ratio of the thermoplastic resin. As a result, reduction of swelling properties while maintaining the conductivity of the charging roller is obtained, and an effect of no generation of image defects caused by the change in the space between the charging roller and the image carrier due to the environment is obtained.
However, obtaining constant stable high surface precision quality without variation even with cutting processes and grinding processes is difficult with the technology of JP-2009-134050-A. Further, the fiber formed of a polymer having an aromatic skeleton in the molecule is generally expensive and increases the cost of the conductive member. Keeping down the cost is desired.